Conventional silicidation techniques employ nickel (Ni)/Pt as a sputter target. The Pt serves as a nickel silicide (NiSi) stabilizer and as an etch stop for a subsequent contact etch. After the NiSi formation sequence of sputtering Ni/Pt and performing a first rapid thermal anneal (RTA), a sulfuric acid-hydrogen peroxide mixture (SPM) is applied to wet strip unreacted Ni. Since SPM fails to strip unreacted Pt, after a second RTA, concentrated Aqua Regia (1:4) (nitric acid (HNO3) plus hydrochloric acid (4HCl)) is employed to strip the unreacted Pt. The HCl yields chloride ions which react with Pt according to formula 1 to form chloroplatinate ion (PtCl6), which in turn is soluble and forms chloroplatinous acid (H2PtCl4).Pt4+(aq)+6 Cl−(aq)→PtCl62 (aq)  Formula 1
The Pt strip process is only effective if the Pt is not bound and can freely react to form the chloroplatinate. If the Pt has formed an organic or oxygenic complex (Pt—C or Pt—O), the reaction with Aqua Regia will not occur, and Pt will remain on the wafer. Furthermore, in addition to the chloroplatinous acid, nitrosoplatinic chloride ((NO2)2PtCl4) is produced, which is insoluble and may result in redeposition. The aforementioned residues lead to significant yield loss, since they are large and cause contact-to-gate shorts.
Pt excursions have occurred in waves. Currently, Pt excursion is observed more significantly on 45 nm technology, typically affecting all wafers of a lot. It was also observed that 32 nm technology has experienced Pt residues. Attempts to eliminate Pt excursions have been applied only when scanned wafers have been affected. However, since only a sample of wafers are scanned, there is a high risk of Pt residues going undetected out of the NiSi module.
To prevent organic material (out of the cleanroom air or minienvironment of the tool) from reaching the wafers and forming Pt—C complexes, organic filters have been installed in RTA tools. Such filters initially appeared to reduce the number of Pt excursions, but recurrences of Pt excursions have been observed even after replacement of the organic filters. Pt excursions may not necessarily be detected after the second RTA, but become pronounced after the spacer shaper etch (etching back the spacers to increase the spaces between the gates for strain liner deposition) passivation clean downstream.
Attempts to eliminate Pt excursions also include a Pt rework procedure including three steps: 1) introduce oxygen plasma (or O2 flash) to break the Pt—C bonds of the Pt—C complex; 2) perform an SPM clean to wet strip the organic residues; and 3) apply Aqua Regia to form chloroplatinate from non-bonded Pt. Such a rework has proven to be 99% effective as long as Pt—O complexes have not already formed. However, in cases where Pt is already present as Pt—O before the rework procedure, the rework is ineffective. Also, applying the rework sequence directly after the second RTA is impossible, since the O2 flash creates Pt—O complexes which are not removable by the Aqua Regia.
A need therefore exists for methodology enabling removal of Pt excursions on all wafers while maintaining a high through-put.